Pesticide volatilization from soil and plant surfaces: Measurements at different scales versus model predictions

Simulation of pesticide volatilization from plant and soil surfaces as an integral component of pesticide fate models is of utmost importance, especially as part of the PEC (Predicted Environmental Concentrations) models used in the registration procedures for pesticides. Experimentally determined volatilization rates at different scales were compared to model predictions to improve recent approaches included in European registration models. To assess the influence of crucial factors affecting volatilization under well-defined conditions, a laboratory chamber was set-up and validated. Aerodynamic conditions were adjusted to fulfill the requirements of the German guideline on assessing pesticide volatilization for registration purposes. Determination of soil moisture profiles of the upper soil layer illustrated that a defined water content in the soil up to a depth of 4 cm could be achieved by water-saturation of the air. Cumulative volatilization of $^{14}$C-parathion-methyl ranged from 2.4% under dry conditions to 32.9% under moist conditions revealing a clear dependence of volatilization on the water content in the top layer. At the semi-field scale, volatilization rates were determined in a wind-tunnel study after soil surface application of pesticides to gleyic cambisol. The following descending order of cumulative volatilization was observed: chlorpyrifos > parathion-methyl > terbuthylazine > fenpropimorph. Parameterization of the models PEARL (Pesticide Emission Assessment at Regional and Local Scales) and PELMO (Pesticide Leaching Model) was performed to mirror the experimental boundary conditions. Model predictions deviated markedly from measured volatilization fluxes and showed limitations of current volatilization models, such as the uppermost compartment thickness having an enormous influence on predicted volatilization losses. Moreover, the impact of soil moisture on volatilization from soil was not reflected by the model calculations. Improvements of PELMO, including the temperaturedependence of water-air partitioning, the reduction of the compartment size of the top layer and the introduction of a moisture-dependent sorption coefficient, contributed to a more realistic reflection of experimental findings, especially at the initial stage of the studies. Studies on volatilization from plants included a field study and a wind-tunnel study after simultaneous application of parathion-methyl, fenpropimorph and quinoxyfen to winter wheat. Parathion-methyl was shown to have the highest volatilization during the windtunnel study of 10 days (29.2%). Volatilization of quinoxyfen was about 15.0%, indicating a higher volatilization tendency in comparison with fenpropimorph (6.0%), which may be attributed to enhanced penetration of fenpropimorph counteracting the volatilization process. A mechanistic approach using a laminar air-boundary layer concept for the consideration of volatilization from plant surfaces was adjusted and calibrated on the basis of a series of wind-tunnel studies. Calibration of the thickness of the air-boundary layer and the rate coefficients of phototransformation and penetration into the leaves allowed the implementation of this description in PELMO and enabled the simultaneous estimation of volatilization from plants and soil. The need to determine critical factors affecting volatilization, especially phase partitioning coefficients, resulted in the development and validation of a novel chamber system for measurements of the temperature dependence of the soil-air partitioning of fenpropimorph. Additional batch studies allowed for the quantification of the general tendency of pesticides towards enhanced soil sorption after lowering the temperature